Conventionally, when a ring gear is to be joined to a gear case of a differential gear, bolts are used to fasten them. However, such bolt fastening would cause problems with an increase in weight due to the weight of bolts and the weight of flanges placed to overlap each other, and further with an increase in machining cost due to the necessity of tapping process and hole-forming process.
Therefore, Patent Documents 1 and 2 listed below disclose other joining methods. Patent Document 1 discloses a joining method in which a gear case and a ring gear constituting a differential gear are placed to overlap one on the other, and a soldering material is interposed on their joint surfaces to join them. To be concrete, in this joining method, while the ring gear is pressed against the gear case, a current is applied to electrodes placed in contact with the gear case and the ring gear, and heat generated by contact resistance between the joint surfaces melts and diffuses the soldering material. Patent Document 2 discloses a laser welding performed by placing an outer peripheral end portion of a flange member in contact with an inner periphery of a ring gear. Specifically, the ring gear is fitted on the flange member and a laser beam is irradiated to a weld part, which corresponds to their peripheral surfaces contacting with each other, from both sides in an axial direction of the ring gear.